1. Field of the Invention
The present invention relates in general to an electric device protecting structure for use in a motor vehicle, and more particularly to a watertight casing structure for protecting an electric device. More specifically, the present invention is concerned with an electric device protecting structure by which a sensitive electric device installed therein is protected from being affected by abnormal temperature, abnormal humidity and dusts.
2. Description of the Prior Art
In a motor vehicle, various electric devices are mounted, some of which are arranged in an engine room. However, since the engine room has therein an engine serving as a heat generator and is exposed to the outside through its opened bottom, the environment in the engine room is not suitable for such electric devices. Thus, in case of very sensitive electric devices which are for example an engine control module (ECM) or the like, it is usual to provide in the engine room so-called electric device protecting structures which house therein the electric devices.
FIG. 4 shows one known example of such electric device protecting structures, which is disclosed in Japanese Patent First Provisional Publication 9-207691. In the protecting structure of this example, there is employed a measure for protecting the electric device from heat generated from the engine. That is, a double wall casing structure 4 is provided at a certain position of the engine room, which comprises an inner case 1 and an outer case 2 between which an air flow passage 3 is defined. The air flow passage 3 is connected through a pipe or the like to an outside air intake opening exposed to the outside of the vehicle. An engine control module 5 which is sensitive to heat is installed in the inner case 1. When the vehicle runs, outside air is forced to flow in the air flow passage 3 while cooling the engine control module 5 in the inner case 1.
FIG. 5 shows another known example of the electric device protecting structures, which is disclosed in Japanese Patent First Provisional Publication 8-99590. That is, in this example, a heat sensitive electric control device 7 is installed in a cowl box 6 that is provided at an upper part of the engine room. Due to its isolated structure, the interior of the cowl box 6 is less affected by heat in the engine room and thus the interior is a suitable place for protecting the electric control device 7 from heat of the engine room. However, since the interior of the cowl box 6 is substantially exposed to the outside through openings and thus easily collects rainwater and the like, the electric control device 7 is constructed to have a sealed structure.
However, due to their inherent constructions, the above-mentioned two known examples have failed to provide users with satisfaction because of the following reasons. In the first example of FIG. 4, due to provision of the double wall structure, the protecting structure is bulky in size and thus largely occupies the limited space of the engine room. In the second example of FIG. 5, the sealed structure tends to lose its sealing ability quickly. That is, when energized, the electric control device 7 generates heat and thus heats the air in the sealed structure, and when deenergized, the electric control device 7 stops generating heat and thus cools the air in the sealed structure. The ON/OFF operation of the electric control device 7 thus induces expansion and contraction of air in the sealed structure causing higher pressure and lower pressure in the sealed structure. This phenomenon however applies the sealed structure with repeated stress. Thus, when the sealed structure is left in such severe condition, the sealing ability of the sealed structure becomes poor quickly.